C−H Bond Activation of Heteroarenes Mediated by a Half-Sandwich Iron Complex of N-Heterocyclic Carbene

Abstract

Half-sandwich iron complexes of N-heterocyclic carbenes, Cp*Fe(L(R))Cl (2a; L(Mes) = 1,3-dimesityl-imidazol-2-ylidene, 2b; L(iPr) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Cp* = eta(5)-C5Me5), have been synthesized by the reaction of Cp*Fe{N(SiMe3)2} (1) with the corresponding imidazolium salts. Treatment of 2a with either methyllithium or phenyllithium replaces the chloride with either a methyl or a phenyl group, generating Cp*Fe(L(Mes))R (3a; R = Me, 3b; R = Ph). These complexes, in turn, undergo cyclometalation at elevated temperatures, and Cp*Fe{kappa2-(C,C)-L'(Mes)} (4; L'(Mes) = CH2C6H2-3,5-Me2-2-(3-mesityl-imidazol-2-ylidene-1-yl)) was isolated. On the other hand, methylation of 2b at room temperature leads directly to the formation of a cyclometalated complex, Cp*Fe{kappa2-(C,C)-L'(iPr)} (6; L'(iPr) = CH2CH(CH3)(3-isopropyl-4,5-dimethylimidazol-2-ylidene-1-yl)). The Fe(II) center of 6 traps atmospheric dinitrogen reversibly to produce a dinuclear end-on N2 complex [Cp*Fe{kappa2-(C,C)-L'(iPr)}]2(mu-eta(1):eta(1)-N2) (7). Complex 6 also promotes C-H bond activation of thiophene, furan, benzothiophene, and benzofuran at room temperature. In these reactions, C-H bond cleavage occurred exclusively at the 2-position of the rings, generating Cp*Fe(L(iPr))(2-C4H3E) (8; E = S, 9; E = O) and Cp*Fe(L(iPr))(2-C8H5E) (10; E = S, 11; E = O), while C-H cleavage took place mainly at the 4-position in the case of pyridine. Coupling reactions between heteroarenes and catecholborane (HBcat) can be carried out by treatment of 6 with heteroarenes followed by the addition of excess HBcat, giving rise to 2-boryl-heteroarenes and the borohydride complex Cp*Fe(L(iPr))(H2Bcat) (14).

N‐Heterocyclic Carbene Piano‐Stool Iron Complexes as Efficient Catalysts for Hydrosilylation of Carbonyl Derivatives

“…[17,18] Our interest in iron catalysis [19] and especially hydrosilylation [20,21] prompted us to examine the possibility of using NHC well-defined iron complexes in the hydrosilylation reaction. The report by Nikonov [22] using cationic [CpFe(phosphine)] complexes and the pioneer works from Brunner [23] oriented us towards piano-stool iron carbene complexes to catalyze such reduction reactions.…”

“…The first examples of mono-carbene complexes were reported in 2003 by Guerchais [13] which was followed by examples from Albrecht [14] , Llewellyn, [15] and more recently the groups of Cesar and Lavigne [16] and Ohki andTatsumi. [17] Up to now, they were used in only very few examples as catalysts. [17,18] Our interest in iron catalysis [19] and especially hydrosilylation [20,21] prompted us to examine the possibility of using NHC well-defined iron complexes in the hydrosilylation reaction.…”

Reactions of [FeL2(CH3CN)2](PF6)2 (L = N-pyrimid-2-ylimidazol-ylidene) with N-, P-, O-, and S-donors and its catalytic activity

“…Iron-SIPr (saturated 1,3-bis(2,6-diisopropylphenyl) imidazolylidene) fluoride could catalyze selective biaryl coupling [13]. A half-sandwich iron-NHC complex was reported as an efficient catalyst for CH bond activation/borylation of furans and thiophenes [14,15], hydrogen transfer reactions [16], and hydrosilylation of carbonyl derivatives [17]. Several iron complexes, including mononuclear complexes containing monodentate [18][19][20] or polydentate NHC ligands [21][22][23][24][25], iron clusters [26][27][28], and polymeric iron complexes [29], have been recently reported.…”

The Development of Ru(II) and Fe(II) Complexes for C-H and H2 Bond Activation

“…Homogeneous transition metal complexes can be tailored to selectively activate stronger C-H bonds such as terminal C-H bonds of C3 and greater alkanes as well as arene C-H bonds in the presence of weaker benzylic C-H bonds. 26,31,32 This is an attractive trait when hydrocarbons functionalized at primary carbons are the desired products. 26,32 However, an industrially viable catalyst for hydrocarbon functionalization would have to exhibit desired reactivity at ~200 °C and have a turn over frequency (TOF) of approximately one turn over (TO) per second.…”

“…26,31,32 This is an attractive trait when hydrocarbons functionalized at primary carbons are the desired products. 26,32 However, an industrially viable catalyst for hydrocarbon functionalization would have to exhibit desired reactivity at ~200 °C and have a turn over frequency (TOF) of approximately one turn over (TO) per second. 33,34 There are several examples of homogeneous transition metal catalysts that have been implemented on an industrial scale, including use in the Wacker process, the Monsanto acetic acid process, and hydroformylation.…”